July Atlantic hurricane outlook

It's mid-July, and we have yet to see a named storm form in the Atlantic this month. The computer models are not predicting any development through at least July 20, and if we make it all the way to the end of the month without a named storm forming, it will be the first July since 2009 without a named storm. Since the current active hurricane period began in 1995, 13 of 17 years (76%) have had a named storm form during July. The busiest July occurred in 2005, when five named storms and two major hurricanes formed. These included Hurricane Dennis and Hurricane Emily--the strongest hurricanes ever observed so early in the season. Only eight major hurricanes have formed in July since record keeping began in 1851. As seen in Figure 1, most of the last half of July activity occurs in the Gulf of Mexico and waters off the Southeast U.S. coast. These type of storms form when a cold front moves off the U.S. coast and stalls out, with the old frontal boundary serving as a focal point for development of a tropical disturbance (as happened for Alberto, Beryl, Chris, and Debby in 2012.) There will be at least two cold fronts moving off the U.S. Mid-Atlantic coast over the next two weeks. The first of these fronts will push offshore around July 20, and we will need to watch the waters offshore of North Carolina for development then. Formation potential will be aided by ocean temperatures that are about 0.7°C (1°F) above average along the U.S. East Coast.

Figure 1. Tracks of all tropical storms and hurricanes 1851 - 2006 that formed July 16-31. The U.S. coast from North to Texas are the preferred strike locations. Only a few storms have formed in the tropical Atlantic and Caribbean in July. Wind shear is typically too high and SSTs too cool in July to allow African waves in the Caribbean and tropical Atlantic to develop into tropical storms. However, a few long-track "Cape Verdes" hurricanes have occurred in July, spawned by tropical waves that came off the coast of Africa. African tropical waves serve as the instigators of about 85% of all major hurricanes.

Sea Surface Temperatures: slightly above averageThe departure of Sea Surface Temperatures (SST) from average over the tropical Atlantic between Africa and Central America was about 0.3°C above average during June (Figure 3.) This figure has not changed much over the first two weeks of July. These temperatures are not warm enough to appreciably affect the odds of a July named storm or hurricane. The strength of the Azores-Bermuda high has been near average over the past two weeks, driving near-average trade winds. The latest 2-week run of the GFS model predicts continued average-strength trade winds through late-July, so SSTs should remain about 0.3°C above average during this period, due to average amounts of cold water mixing up from below due to the wind action on the water.

Figure 3. Sea Surface Temperature (SST) departure from average for July 12, 2012. SSTs were 0.3°C above average over the tropical Atlantic's Main Development region for hurricanes, from Africa to Central America between 10° and 20° North Latitude. Note the large region of above average SSTs along the Equatorial Pacific off the coast of South America, the hallmark of a developing El Niño episode. Image credit: NOAA/NESDIS

El Niño on the way?For two consecutive weeks, ocean temperatures 0.5 - 0.6°C above average have been present in the tropical Eastern Pacific, which is right at the threshold for a weak El Niño episode. NOAA's Climate Prediction Center has issued an El Niño Watch, and gives a 61% chance that El Niño conditions will be present during the August - September - October peak of the Atlantic hurricane season. The likely development of a full-fledged El Niño episode means that Atlantic hurricane activity will probably be suppressed in 2012, due to the strong upper-level winds and high wind shear these events typically bring to the tropical Atlantic.

Figure 4. Sea Surface Temperature (SST) departure from average for the the equatorial Eastern Pacific (the area 5°N - 5°S, 120°W - 170°W, also called the "Niña 3.4 region"). El Niño conditions exist when the SST in this region rises 0.5°C above average. As of July 9, 2012, SSTs in the Niño 3.4 region had risen to 0.5°C above average. To be considered an "El Niño episode", El Niño conditions must occur for five consecutive months, using 3-month averages. Image credit: NOAA Climate Prediction Center.

Wind shear: above averageWind shear is usually defined as the difference in wind between 200 mb (roughly 40,000 foot altitude) and 850 mb (roughly 5,000 foot altitude). In most circumstances, wind shear above 20 knots will act to inhibit tropical storm formation. Wind shear below 12 knots is very conducive for tropical storm formation. High wind shear acts to tear a storm apart. The jet stream has two bands of strong high-altitude winds that are currently bringing high wind shear to the Atlantic. The southern branch (subtropical jet stream) is bringing high wind shear to the Caribbean, and the northern branch (polar jet stream) is bringing high wind shear to the waters offshore of New England. This configuration often leaves a "hole" of low shear between the two branches, off the Southeast U.S. coast and over the Gulf of Mexico. The jet stream is forecast to maintain this two-branch pattern over the coming two weeks. Wind shear has been about 10 - 20% higher than average over the first two weeks of July, and is predicted to be mostly above average for the coming two weeks. This will cut down on the odds of a July storm.

Figure 5. Vertical instability over the Caribbean Sea in 2012 (blue line) compared to average (black line.) The instability is plotted in °C, as a difference in temperature from near the surface to the upper atmosphere. Thunderstorms grow much more readily when vertical instability is high. Instability has been lower than average, due to an unusual amount of dry air in the atmosphere, reducing the potential for tropical storm formation. Image credit: NOAA/NESDIS/CIRA.

Dry air: above averageAs seen in Figure 5, there has been an unusual amount of dry, stable air in the Caribbean this year creating low levels of vertical instability. This has occurred due to a combination of dry air from Africa, and upper-atmosphere dynamics creating large areas of sinking air that dry as they warm and approach the surface. The Gulf of Mexico and tropical Atlantic between the coast of Africa and the Lesser Antilles have also seen low vertical instability this summer. June and July are the peak months for dry air and dust coming off the coast of Africa, and the Saharan dust storms have been quite active over the past two weeks. Expect dry air to be a major deterrent to any storms that try to form in the tropical Atlantic during July.

Steering currents: averageThe predicted steering current pattern for the next two weeks is a typical one for July. We have an active jet stream bringing many troughs of low pressure off the East Coast of the U.S. These troughs are frequent enough and strong enough to recurve any tropical storms or hurricanes that might penetrate north of the Caribbean Sea. Steering current patterns are predictable only about 3 - 5 days in the future, although we can make very general forecasts about the pattern as much as two weeks in advance. There is no telling what might happen during the peak months of August, September, and October--we might be in for a repeat of the favorable 2010 and 2011 steering current pattern, which recurved most storms out to sea--or the unfavorable 2008 pattern, which steered Ike and Gustav into the Gulf of Mexico.

Summary: a below average chance of a July tropical stormGiven that none of the computer models are forecasting tropical storm formation in the coming seven days, SSTs are only slightly above average, and wind shear and vertical stability are above average, I'll go with a 30% chance of a named storm forming in the Atlantic during the remainder of July.

Figure 6. Hurricane Emilia over the Eastern Pacific at 20:35 UTC July 10, 2012. At the time, Emilia was a Category 3 hurricane with 125 mph winds. Emilia peaked earlier in the day as a Category 4 storm with 140 mph winds--the strongest hurricane in the East Pacific so far in 2012. Image credit: NASA.

An active Eastern Pacific hurricane seasonIt's been a very active start to the Eastern Pacific hurricane season, where we've already had six named storms, four hurricanes, and three intense hurricanes. A typical season has 4 named storms, 2 hurricanes, and 0 intense hurricanes by July 14. The formation of Tropical Storm Fabio on July 12 marks the 4th earliest formation of the Eastern Pacific's season's sixth storm. The record is held by the year 1985, when the season's sixth storm formed on July 2. Record keeping began in 1949.

Have a great weekend, everyone, and I'll be back Monday with a new post.

Don't most that organize in that area go fishing? I know Beryl had a landfall in Florida, Just wondering.

Along trough splits yeah, but if the trough makes it down all the way into the gulf and sits stationary for days you can get development in the gulf, like Hurricane Gabrielle in 2001. We'll have to keep looking for trends to see if the GFS pushes the trough further south and something develops on the tail of it.

Quoting GTcooliebai:There is a chance for the wave that just emerged off Africa for development, but it will have to be quick. The High Pressure Ridging is the driving force for the SAL and it has weakened a bit, but will be making a return and this time emerging from Europe or just of the coast from there.

Looking down stream, a trough split looks to occur and may shoot off areas of low pressure and if one is lucky enough with good conditions aloft we may get Ernesto.

Don't most that organize in that area go fishing? I know Beryl had a landfall in Florida, Just wondering.

Sometimes there is a confusion about the SAL or Saharan Air Layer. It is actually composed of actually 3 layers. A top layer,which is hot and dry; the mid layer, which is cooler air from the Atlantic; and a third layer which is cooler dense air from the Atlantic.

Sometimes dust storms mix with this layer. Actually, Most scientists agree that the SAL actually aid in the formation of some hurricanes by using these components to cause a spin. Heavy amounts of dust are not always in the SAL.

The SAL can inhibit the development of hurricanes,because of the level of moisture in the mid-level. If it is too dry, development becomes less likely.

Here is an example of Hurricane Erin forming within a strong SAL environment.

Quoting floridaboy14:we are getting el nino. the problem is the models showed it developing already in july however now we know thats not the case. i said before el nino would develop in the early fallaround september 20th and beyond and my prediction might come true. this is why this season will be slightly active and we will have 12-15 storms

we are getting el nino. the problem is the models showed it developing already in july however now we know thats not the case. i said before el nino would develop in the early fallaround september 20th and beyond and my prediction might come true. this is why this season will be slightly active and we will have 12-15 storms

It's pretty bad here. Woke up to Environment Canada saying that out current condition was "Haze". They don't offer an air quality index so I don't really know how bad it actually was. At least the sun is shining, it does get so bad that it blocks the sun a few times a year.

You may want to watch this site. They aren't able to get their hands on all the Canadian data but they are trying to use everything available to help y'all out.. 13th looked thick in some spots..

It's pretty bad here. Woke up to Environment Canada saying that out current condition was "Haze". They don't offer an air quality index so I don't really know how bad it actually was. At least the sun is shining, it does get so bad that it blocks the sun a few times a year.

The ITCZ will be moistened up in the coming weeks as the larger waves move across so they do pave the way for the ones that follow by providing a better moisture field between the ITCZ and SAL layer. A big SAL outbreak can always dip down and disrupt the chain as some point but it recovers at some point. As far as this aspect of the ITCZ, the Water Vapor loops are a great tool to keep an eye on.

There is a chance for the wave that just emerged off Africa for development, but it will have to be quick. The High Pressure Ridging is the driving force for the SAL and it has weakened a bit, but will be making a return and this time emerging from Europe or just of the coast from there.

Looking down stream, a trough split looks to occur and may shoot off areas of low pressure and if one is lucky enough with good conditions aloft we may get Ernesto.